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He admitted taking part, but claimed he was forced to do it because of threats to kill him and his mother by two Eastern European underworld figures known only as 'Kev' and 'the big boss'.

A jury had failed to reach verdicts in his case in July last year.

Kassaye was found guilty of conspiracy to rob, kidnap and possession of a firearm last summer and three others were convicted of playing a part in the robbery.

Kassaye, of Battersea, south-west London, was jailed for 23 years wile Solomon Beye of St John's Wood, north-west London; Clinton Mogg, of Bournemouth; and Thomas Thomas, of
East Finchley, north London
were jailed for 16 years each.

Kassaye's 'second in command' Beyene, 25, bought the gang's 'dirty' pay-as-you-go phones used in the
robbery, Mogg, 43, provided a flat for make-up work to be done
and Thomas, 46, hired a 7.5-tonne lorry which was used to block
traffic.

Terrifying ordeal: The thieves took shop assistant Petra Ehnar hostage as they escaped to a getaway car

'Life or death': Shop assistant Petra Ehnar feared Craig Calderwood and Aman Hassaye, pictured being held hostage outside the store, would shoot her

Getaway: The thieves make their escape after one of their accomplices blocked traffic with a lorry

Lawyers for Calderwood and Kassaye claimed the raid was an 'inside job' orchestrated by the Graff family and had been staged to capitalise on a huge insurance payout.

But a letter of confession was found inside Calderwood's cell at Belmarsh prison in which he admitted his guilt and apologised to the shop's staff.

The scribbled note, full of spelling errors and addressed to the trial judge, was found among personal papers on an A4 notepad during a routine search of his cell at Belmarsh high-security prison on December 31, 2009.

He wrote: 'Ur honer (sic) I would like to say a few words to you and the court. I am truly sorry for the crime I have committed.

'The man you are going to see in the video that is not me, I am not that person. I dug a big hole for myself in life and now by me admitting my guilt I am now filling it in with myself in it and when you see fit I will grow in to a bigger, better, smarter man.

'I would like to send my sincerest apologies to the staff at Graff store, especially to Martin Leggatt for the traumatic and scary ordeal I must have put them through and once again sorry.'

Never found: The stolen gems were handed to a motorbike rider who has not been identified

Despite the convictions, the crime bosses who masterminded the robbery remain at large and the whereabouts of the stolen diamonds is unknown.

Each stone has been individually laser-stamped with a unique identifying code which cannot be removed and can be checked quickly and easily, but none of the stolen diamonds have surfaced anywhere around the world.

The court heard the robbery took place shortly after 4.40pm on August 6 2009 and was carried out with military precision.

Kassaye and Calderwood had been 'aged' with make-up earlier in the day and arrived in New Bond Street in a black cab.

Impeccably dressed in smart suits they glanced in the Graff window, before approaching security staff and were allowed to enter.

Both men pulled out weapons and ordered staff to the floor, before Miss Ehnar was picked to open the revolving display cases.

Calderwood stood over Mr Leggatt, pointing the gun at his head and threatening him.

He said: I could feel him right over me and I was not going to move or flinch at all because I truly, genuinely totally feared for my life.'

Terror: Staff were held at gunpoint during the dramatic 2009 incident

Miss Ehnar, who had only been working at Graff for three-and-a-half months, described it as a 'life or death' situation.

'I expected the worst,' she said. 'I thought if anything goes wrong they might shoot me.' With Kassaye holding a small bag containing the gems, he pressed the gun in to Miss Ehnar's back and marched her from the store to a waiting BMW estate.

Israeli Etan Azulay, a Graff security guard who had pressed a panic alarm tried to tackle Kassaye but was shot at from almost point blank range.

He fired further shots when the getaway car crashed in to a taxi in nearby Stafford Street as brave passer-by Robert French tried to intervene.

Calderwood was also seen firing from the back of the BMW and was identified because of his white socks and short trouser legs, before both men ran to a waiting Mercedes.

Inside the abandoned BMW police found a loaded shotgun lying on the back seat and one of the gang's phones which had slipped between the driver's seat and handbrake.

Their escape had been made easier as other gang members blocked roads with a transit van, Renault box lorry and an Audi Q7.